Correlation and prediction of reservoir quality variability/heterogeneity within shallow carbonate ramp deposits of the Arab Formation in an Abu Dhabi offshore field has previously proved challenging. This study, commissioned by ADMA-OPCO, presents the results of integrated sedimentology and reservoir quality assessment of the A to D members of the Arab Formation.

A consistent set of Dunham-based lithofacies have been used to characterise ca. 6,800 ft of the Arab Formation in 30 partially cored wells. These sub-bed/bed-scale lithofacies are up-scaled and grouped into a series of lithofacies associations based on genetically related sedimentary structures, textures and allochem assemblages to provide an understanding on the depositional evolution of the Arabian Platform. These lithofacies associations, coupled with the broad lithological characterisation (anhydrite, dolomite and limestone), also provide fundamental descriptors for subsequent reservoir quality analysis.

Calibration of lithofacies associations with the open-hole logs enabled extension of the sedimentological framework into the uncored intervals/wells. However, this process demonstrated that there is considerable uncertainty in recognising facies associations using wireline logs alone, and this reduced confidence has implications for the field-scale sedimentological model. The facies population of the uncored intervals was assisted by the generation of a revised sequence-stratigraphic framework, derived mainly from the recognition of key surfaces and vertical facies stacking patterns derived from core measurements. The sequence-stratigraphic framework captures a hierarchal scale of eleven fourth-order cycles over three third-order cycles each marking a transgressive (flooding) and regressive event developed over the formation-scale shallowing-upwards trend.

This sedimentological database enabled production of facies trend maps for each of the fourth-order cycles, that provide an understanding of the lateral facies variability across the field. These GIS trend maps are generated from the interrogation of thickness data of lithofacies associations from both cored and uncored intervals. Through the application of percentage ‘cut-off values to the thickness data, the lateral facies trends are plotted using sedimentological principles and the understanding gained from the sequence-stratigraphic and depositional models. The resulting GIS shape data provides important constraints on geobody geometry and lateral facies development for the update of the static geological model.

Reservoir quality analysis, based on conventional core analyses data coded to lithofacies and lithofacies associations, provides an assessment on the influence of primary pore fabrics and subsequent diagenetic modifications, such as dolomitisation, on pore system development. Furthermore, the lateral distribution of reservoir quality within the limits of the third- and fourth-order sequence-stratigraphic cycles is investigated and mapped to provide a better understanding of the impact of the sedimentological and diagenetic influences on reservoir behaviour and distribution. Recent proprietary diagenetic studies provided by ADMA-OPCO have been considered, although a full diagenetic study is beyond the remit of this project.

In addition, previously established rock types supplied by ADMA-OPCO have been tested against lithofacies associations and lithological descriptors generated by this study. The tests evaluated the robustness and relationship to the sedimentological model so as to help constrain the distribution and predictability of the reservoir rocks on a field-wide basis for future static reservoir models. Neutron/density log responses are coded to core-based lithofacies associations to test the relationship between porosity, cementation and facies with the aim of producing a diagnostic method for identifying intervals of improved reservoir quality in uncored intervals elsewhere in this field. However, a full and detailed petrophysical evaluation does not form part of the remit of this study and would be required to confirm initial observations.

Summary

The Makhul is an unconventional formation in Kuwait. This presentation will compare the Makhul Formation in Kuwait with the Makhul Formation in other Arabian Plate countries and add insights to the differences of the sequence stratigraphy and sedimentology to other studied areas. The sedimentology study provides additional insight into the environments of deposition and diagenesis of the Makhul in Kuwait to aid in the complete understanding of this formation. Core, cuttings, thin sections, logs will be evaluated to provide the study with comprehensive insight.

Makhul Formation

The shallow-marine carbonate sediments of the Makhul, Minagish and Ratawi formations form the lower part of the Thamama Group as defined in Saudi Arabia. The three formations in Kuwait are informal lithostratigraphical units defined and dated by calibration with surface exposures and wells in neighboring Saudi Arabia, Iraq and Iran. The Makhul in Kuwait has had many oil and gas shows without proper conclusions. Therefore, the geological study of this formation could not be conducted with accuracy.

Stratigraphy and Depositional Setting

The Makhul Formation is equivalent to the Sulaiy Formation in Saudi Arabia and is of Late Jurassic (Tithonian) to Early Cretaceous (Berriasian) age. The recognition of the boundaries and sub-divisions is problematical and can be improved with further biostratigraphical data and analysis. The most recent study in 2012 using bioassemblages (local biozones) approximate the Makhul Formation as a radiolarite assemblage, which helped identify horizontal and vertical zones. West of the Kuwait Arch a radiolarite facies (radiolarian, calcispheres, sponge spicules, etc.) with significant amount of organic matter represents a deep-marine, euxinic, low-energy environment below storm wave base. The occurrence of calpionellids and tintinids indicate a deeper open-marine regime in the offshore areas east of the Kuwait Arch, in the wells west of the arch they are almost absent. This biozone can be divided into two sub-biozones:

(1) a lower sub-biozone characterized by the virtual absence of an in-situ fauna/flora and dominantly lime mudstones; and

(2) an upper sub-biozone characterized by a diverse but sparse in number in-situ benthonic and planktonic fauna/flora and with an upward increase in the occurrence of wackestones and packstones.

The two sub-biozones are believed to correspond to two third-order sequences as delineated by Sharland et al. (2001) with associated third-order maximum flooding surfaces corresponding to MFS J110 and MFS K10 and is in accordance with latest sedimentological studies (see below).

The base of the Makhul Formation is not an easy pick to make and traditionally has been made at the highest bedded anhydrite/carbonate of the Hith Formation. In basinal/distal areas (in the intra-shelf basins) in Iraq the base of the Makhul is thought to be essentially gradational and conformable. In proximal/marginal areas within the flank areas of the intra-shelf basins (Kuwait) the boundary is considered an unconformity (Sharland et al., 2001). It is often denoted by a major hard ground – the intra-Late Jurassic Unconformity AP8/AP7 boundary, 149 Ma, intra-Tithonian.

The presence of coccolith species Conusphaera Mexicana minor in the lower part of Makhul Formation (Burgan) locate the Jurassic/Cretaceous boundary within this formation. The top of the Makhul Formation is poorly defined on wireline logs and is primarily based on biostratigraphic data. The top of the Makhul is defined by a downward facies change from relatively clean limestones of the Minagish to argillaceous limestones of the Makhul characterized by common sponge spicules and radiolaria. The top and base of the formation are considered to be diachronous, reflected in prograding and retrograding lateral facies changes in an outer to inner ramp setting. The carbonate ramp is interpreted to deepen to the north and shallow to the southwest.

Facies and Depositional Environments

The Makhul Formation was deposited in the mid to outer parts of a gently inclined homoclinal ramp formed within an initially restricted, anoxic intra-shelf basin following transgression at the top of the Hith. The overlying Minagish Formation was deposited in mid to inner-ramp environments and represents progradation of the ramp from the southwest to the northeast. The contact between the Makhul and Minagish formations is transitional and diachronous due to lateral facies change. The cored facies described are consistent with an overall deepening to the NE with proximal outer-ramp and mid-ramp noted in Mutriba and Minagish fields.

Two main facies associations are identified. The dominant facies association comprises laminated argillaceous and locally bituminous lime mudstones. They are interbedded with thin, relatively pure wackestone to grainstone turbidite beds with scoured basal contacts, internal grading and rippled tops. In the absence of evidence of a slope facies, these turbidites are considered to be distal tempestites associated with storm activity in shallower parts of the ramp. Bioturbation is limited to Chondrites and Helminthopsis indicating a distal Zoophycos ichnofacies. Skeletal fragments are rare except in the lower part of the Makhul where accumulated pelagic fauna including ammonites and Saccocoma are preserved.

In the lower part of the Makhul in northwest Raudhatain field the facies is very bituminous and characterized by a high gamma-ray response due to high uranium content. This facies association is interpreted as outer ramp below storm wave base with intervals containing relatively abundant turbidite beds categorized as proximal outer ramp. One interval in Mutriba with common thin grainstone turbidites is sharp-based and may reflect a forced regression of more proximal outer ramp above distal outer ramp.

The second main facies association comprises relatively massive, homogenous bioturbated grainy lime mudstones, wackestones and occasional packstones with argillaceous interbeds. Bioturbation is relatively diverse with Zoophycos, Paleophycus, Planolites and Teichichnus in addition to Chondrites and Helminthopsis indicating a more proximal ichnofacies. Skeletal fragments are minor and include few mollusc fragments, benthonic forams and echinoderm fragments. This facies association is interpreted to be mid-ramp and deposited close to but generally above storm-wave base.

Reservoir Potential

Reservoir potential of the Makhul Formation is poor. Outer-ramp facies are dominantly tight with no visible porosity identified within the lime mudstone facies or interbedded grainy turbidites, where primary porosity is occluded by calcite cement. Mid-ramp facies locally show very poor reservoir potential with trace matrix intercrystalline porosity noted in argillaceous mid-ramp. This microporous interval is reflected in increased neutron response confirming minor amounts of porosity. A slight increase in deep and shallow resistivity values over this interval suggest minor hydrocarbon content.

Fractures are generally closed or calcite cemented. Calcite-lined and bitumen-plugged fractures are relatively common in the lower part of the Makhul. Most fractures are subvertical and are of limited vertical extent ranging up to ca. 2 feet in length. Most are associated with relatively brittle beds and taper out at contacts with less indurated argillaceous beds. These fractures are also limited to well-indurated beds.

Middle Jurassic to Early Cretaceous petroleum systems of the Arabian Plate

What are arguably the most prolific petroleum systems in the world are driven by multiple world-class source rocks. These highly organic-rich rocks account for most of the discovered oil and gas on the Arabian Plate. The source rocks were deposited in widespread shallow-marine intra-shelf basins and deep-marine basins. They differ in age between basins and are not correlative with global occurrences of organic enrichment.

Burial under Mesozoic and Cenozoic strata has caused the source rocks to generate oil and gas. Timing of generation differs mainly by variations in sedimentary load; differences in the kinetics of generation and regional heat flow play a lesser role. In the west of the Arabian Plate, generation occurred under Cretaceous and Paleogene sedimentary load, whereas in and near the Zagros Foldbelt generation continues under recent syntectonic sedimentary load.

The most organic-rich source rocks are carbonates containing mainly marine (Type-I/II) organic matter that has generated sulfur-bearing oil and gas. The gas in some carbonate reservoirs is further enriched in H2S derived from thermal-sulfate reduction. Clay-rich source rocks contain a mixture of marine and land-plant (Type II/III) organic matter and have generated a mixture of low-sulfur oil and gas. Gas is more common also where the source rocks are deeply buried and more thermally mature.

Low-angle structural dip of highly continuous strata allowed oil and gas to migrate laterally for greater than 150 km from areas of generation to traps, limited only by the extent of structural dip and continuous seal. Intervals of widespread evaporite and shale acted as barriers to vertical migration. Oil and gas migrated to younger rocks through gaps in migration barriers caused by non-deposition or erosion, including localized karst dissolution, or by faults that pierce these migration barriers, commonly where they are thin. Oil and gas generated from Jurassic and Early Cretaceous source rocks are found in many Jurassic, Cretaceous and Cenozoic reservoirs, but rarely occur in older rocks.

The Arabian Plate is reservoir rich, with multiple reservoir-seal pairs for oil and gas accumulations. Most reservoir rocks are shallow-marine carbonates and paralic sandstones, but non-marine and deep-marine sedimentary rocks and even fractured basement are reservoirs in some basins. Shallow burial and limited cementation in the subsurface have favored preservation of porosity and some superb reservoir quality.

Most traps are structural. Compressional thrusted anticlines dominate the Zagros Foldbelt, whereas compressional anticlines with reactivated basement-involved faults occur on the Arabian Platform. Halokinesis of deeply buried Neoproterozoic–Cambrian salt forms traps, especially in the offshore Gulf and in Oman. Extensional faulted anticlines form traps in rift basins, notably in Syria and Yemen. The few discovered stratigraphic traps include truncation pinchouts and reefal buildups.

Stratigraphy and depositional history of the Marib-Jawf Basin, Yemen

The sedimentary fill of the Marib-Jawf Basin is Jurassic and Lower Cretaceous. Initial marine transgression occurred in the Middle Jurassic, with the Kohlan Formation unconformably overlying Paleozoic sedimentary rocks and Proterozoic basement. These basal fluvial to nearshore-marine sandstones and shales are conformably overlain by intertidal to shallow-marine carbonates of the Saba Formation of Callovian–Oxfordian to Kimmeridgian age.

Extensional faulting formed grabens that deepened during the Kimmeridgian and Tithonian. Up to 6,000 feet (2,000 meters) of deep-marine sediments unconformably overlie and lap onto high margins of the basin. At the base are outer-shelf/slope argillaceous carbonates and shales of the Arwa Formation. These pass upwards into slope and basinal shales with minor limestones and sandstones of the Meem Formation, which in turn are overlain by shales with interbedded sandstones and limestones of the Lam Formation. The sandstones were deposited by turbidity currents from the margins of the basin, where submarine fan deposits are present. Shales of the Meem and Lam formations are organic-rich and are the source rocks for most of the oil and gas in the basin.

A drop in relative sea level occurred in the middle to late Tithonian with restriction of the basin from the open ocean. Three progradational sequences (Yah, Sean and Alif members of the Alif Formation, from base to top) of fluvial-alluvial to deltaic-marine siliciclastic sediments were deposited down the axis of the basin, passing into offshore-marine shales and basinal evaporites to the east. The sandstones are the main reservoirs for oil and gas in the basin.

A transgressive shale at the top of the Alif Formation records marine flooding of the basin, followed by deposition of five sequences of basin-filling evaporites of the Safer Formation. Thick halite beds were deposited subaqueously when the basin was filled with hypersaline water and have thin anhydrite beds at top and base. These evaporites are separated by lowstand fluvial to shallow-marine siliciclastics, including thin, organic-rich shales that are minor source rocks for oil. The evaporites are the primary seal for the reservoirs of the Alif Formation.

Berriasian–Valanginian shelfal-marine shales and limestones of the Azal Formation unconformably overlie the Safer evaporites. The lower part of the Azal Formation is dominantly limestone and shale, the middle is shale with few thin limestones, and the upper unit has limestone, shale and minor sandstone. These are overlain by the Tawilah Group, possibly as old as Valanginian-Early Hauterivian and at least as young as Aptian in parts of the basin, equivalent to the Qishn Formation to the east. The Tawilah Group is dominated by non-marine to shallow-marine sandstones and shales with minor thin dolomite and skeletal-limestone beds. Regional erosion truncates the sedimentary fill of the basin, progressively cutting out more strata toward the west and in places exposing Middle Jurassic rocks at the surface.

The influence of relative sea-level variation on depositional settings of the Late Jurassic Arab Formation in a giant gas field, United Arab Emirates

Detailed core description of multiple wells within a giant gas field in the United Arab Emirates (UAE) has enhanced the understanding of the depositional make-up and possible controls on depositional style for the Arab Formation. In this field, the Arab Formation differs from other areas such as Qatar in that anhydrite and dolomite are scarce in the lower units of the Arab Formation and recognition of any sequence-stratigraphic boundaries is limited. In summary, the limestone-dominated reservoir units combine to reflect a large-scale shallowing-upwards trend from basinal/outer, mid-ramp to inner-ramp depositional settings, based on core-derived facies association analysis. The facies associations are defined according to sedimentological and faunal characteristics and form genetically related, larger-scale units reflecting a low-energy, mid- to outer-ramp depositional setting, which grades sharply into an inner-ramp shoal and shoal complex depositional setting.

Within the low-energy units of the lowest Arab Formation reservoir, mudstones are interbedded with bioclast-rich accumulations of variable thickness and an uncertain origin. The vertical spacing of these units may be a function of random/non-random cyclicity (?Milankovitch) and reflect variation in depositional slope angle and hence slump/debrite deposits or deeper-water faunal communities. The transition from the distal depositional setting to the proximal depositional setting occurs over a relatively narrow zone into oolitic grainstones as the system progrades. The grainstones form a thick sequence of bedded units reflecting both shoal and intershoal areas within an inner-ramp depositional setting, but cyclicity is not apparent in this interval.

The small-scale cyclicity present in the lower units of the Arab Formation may potentially be equal to those that are defined by dolomite-anhydrite cycles in Qatar, but in this area of Abu Dhabi, the overall distal setting has negated the development of these mixed evaporite-carbonate cycles. The implications of this include a lack of intraformational seals and a uniformity of lithofacies (i.e. limited variation), which is also expressed in the more proximal inner ramp deposits and provides for good reservoir potential.

The integration of subsurface and outcrop data allowed the building of a 200 km NW-SE correlation transect across seven oil fields in Abu Dhabi (Figure 1). It reveals possible new concepts for the stratigraphic organization of the Late Jurassic–Early Cretaceous systems in the United Arab Emirates (UAE).

The sedimentological study of a total of 1,272 m of cores from 16 wells led to the definition of 12 sedimentary facies and their calibration to gamma-ray logs. The interpretation of regional seismic lines tied with wells constrained the stratigraphic architecture of the studied interval, including the delineation of major unconformities, sedimentary geometries (clinoforms) and lateral seismic-facies variations. The final NW-SE correlation transect was built by the integration of seismic geometries, core and well-log sedimentary facies, seismic facies, and the results of a regional study of age-equivalent outcrops and subsurface data from the Sultanate of Oman (Lebec, 2004; Dujoncquoy, 2011).

The proposed stratigraphic architecture of the Late Jurassic–Early Cretaceous systems of the UAE records four main stages of evolution.

Late Jurassic Uplift and Erosion to the East

Several stratigraphic features indicate that an uplift occurred in the eastern part of the study area during the Late Jurassic: (1) westward-oriented prograding clinoforms in the Hanifa Formation; (2) truncation of the top Hanifa Formation to the east; (3) complete wedging of the Late Jurassic sequence in the Ras al-Khaimah area (Razin et al., 2013); and (4) westward syn-sedimentary thickening of the Jubaila–Arab D sequence. The proposed correlations in the Arab A, B and C members also indicate a progressive syn-sedimentary wedging of the different depositional sequences towards the east. The wedge is interpreted as related to the syn-sedimentary uplift of the eastern domain, which appears to have been strongly eroded during that time. This tectonic deformation can be considered to have favored restricted conditions and the development of evaporite-carbonate systems (Arab-A,- B and -C members).

Late Tithonian Regional Subsidence Increasing to the East and Major Transgression

After this Late Jurassic deformation, subsidence of the entire platform is recorded by the Late Tithonian transgressive sequence. Eastward increasing subsidence is attested by: (1) tilting of the previous erosional unconformity and underlying sequences; (2) westward onlap of the first high-resolution sequences; and (3) W-E polarity of the new sedimentary system. During the early stage of the Late Tithonian transgression, the depositional profiles were of very low angle, favoring a wide extent of transgressive oolitic facies.

Late Tithonian–Late Berriasian Differential Aggradation and Progradation of the Habshan Carbonate System

After this first cycle, the increased rate of accommodation and carbonate production resulted in strong aggradation of the Habshan lagoonal deposits in the northwestern part of the platform (Figure 2). The lower sedimentation rate to the southeast led to a progressive tilt of the successive depositional profiles, and to the development of relatively steep prograding clinoforms made of oolitic and bioclastic grainstone, deposited in a platform margin environment (“Habshan Oolite”).

The top Habshan surface corresponds to a major exposure surface linked to the Late Berriasian-Early Valanginian lowstand related to an “icehouse” global context. The Thamama F, G and H (“Zakum” equivalent) transgression and aggradation is related to a major sea-level rise that followed the Early Valanginian lowstand. The top Thamama F (top “Zakum”) surface corresponds to another major exposure surface related to the Late Valanginian–Hauterivian lowstand period, interpreted as a result of a regional uplift of the Arabian Plate (“Late Valanginian unconformity”).

The proposed Late Jurassic–Early Cretaceous stratigraphic model provides new concepts for well correlations at both regional and reservoir scale, which may have an impact on reservoir models and prediction. It also allows building a coherent stratigraphic scheme for the Late Jurassic-Early Cretaceous interval from the UAE to the Sultanate of Oman (Razin et al., 2013).

Regional correlation transect illustrating the evolution of geometries and facies within the Late Jurassic-Early Cretaceous Habshan carbonate system in Abu Dhabi, UAE. This transect is based on seismic interpretation integrated with core description and well log analysis.

Regional correlation transect illustrating the evolution of geometries and facies within the Late Jurassic-Early Cretaceous Habshan carbonate system in Abu Dhabi, UAE. This transect is based on seismic interpretation integrated with core description and well log analysis.